It’s raining out there, so let’s go harvest

Reading the topic of this article, one of two things probably comes to mind, the first one might sound somehow like this “let me guess, he’s written about water again” and the second one might sound like “what the hell is he talking about, harvesting while it’s raining? He probably doesn’t know anything about farming cause if he does, he’d know there are different time for every farming activity and harvesting usually comes after the rainy season stops” if you fall in the first category, congratulations to you just won a gold medal for making the easiest guess there is, if you fall in the second category, commiserations to you, maybe next time. Anyway, I am indeed here to talk about water again, specifically rain water harvesting (people in category two would be like “oooh now I get it”) so what is rain water harvesting?

What is Rainwater Harvesting?

This should be easy to define, we probably wouldn’t need the help of any kind of dictionary in defining this as the words alone should be self-explanatory. Rain Water Harvesting is simply the collection and storage of rain water which would then be used for several water related purposes like irrigation, domestic use, drinking and even flood prevention purposes.

Rain water harvesting isn’t a new concept, it’s something which we’ve probably done at one point or the other in our lives, matter of fact it’s been around for quite some time now with it having been in use as far back as the Neolithic age^[1] in various regions such as southwest Asia, Jerusalem, Greece and Rome.

As a Nigerian, it’s something we did growing up but we never really considered it as anything other than an afterthought and why should we? After all, the majority of us get their water from a borehole or are supplied by the water company, sources that are supposedly cleaner and would always be available and hence more reliable, but are they? Let’s talk about water quantity and quality for a while.

Water Quantity and Quality

In my last post, I briefly mentioned how much water we have on earth which is about 70% of the earth’s surface, the earth’s surface area is 510 million km², this means that an astounding 361.13 million km² of earth’s total area is all water (boy that is a lot of water), we should just be able to walk a few miles to the nearest water source and take a cup of water to drink, but you and I know we can’t do that, unless of course you are immune to every waterborne disease known and unknown to man and you can also stand the taste and smell of polluted water, if that is you then congratulations, this article isn’t for you, kindly move along. For the rest of us that can’t do that, please take a seat as I’d be throwing bits of facts and figures your way.

As much water as there is on earth, only 2.5% of it is freshwater, the remaining 97.5% is salty and found in our oceans and inland sea. Of the 2.5% freshwater, about 69% of it is locked up on ice and glaciers while about 30% is found in groundwater sources leaving about 1% available as surface water (fresh water lakes and rivers)^[2] and with that being the major source of water for human consumption on earth, it means that about a fraction (about 0.01%) of earths total water is what we actually have available for our use(I bet our water doesn’t seem so much to you now). Now let’s see how clean our groundwater and surface water really is in comparison to rainwater.

Quality of surface and ground water in relation to rainwater

“Pure water” is a term which is loosely used to describe good quality water around the world, but in reality, there is no such thing as a completely pure water. No matter where your water is gotten from, either ground, surface or rain, they would always contain certain impurities due to the fact that water is a universal solvent which dissolves anything it comes into contact with, so for ground water, even after the water is naturally filtered by the sand on its way to groundwater aquifers, it still dissolves some minerals which it carries on its way and lets not even talk about surface water which is easily polluted and even after treatment would still contain little impurities, as for rain water, it dissolves every substance it comes into contact with in the atmosphere like dusts and gases even before it hits the ground surface. So basically, water of good quality is just water with a very small amount of impurities, small enough to cause no harm to the human body.

The quality of water is therefore determined by measuring what is known as water quality constituents or simply the amount of impurities present in the water. These constituents are quite numerous but are basically grouped into biological constituents, chemical constituents and physical constituents. If I decide to talk about every water quality constituents in each group I would not finish this article so instead, I’ll just pick a few of them and compare the typical concentrations of these water quality constituents in the average ground water, surface water and rain water sources.

Typical concentrations of some water quality constituents in groundwater, surface water and rainwater sources

BOD= biological oxygen demand, TDS= total dissolved solids , pfu= plaque forming units, NTU= Nephelometric turbidity unit, MPN= most probable number

From the above table, it can be seen that we have a clear winner when it comes to the quality of water normally gotten from these three sources. Now let’s see how available groundwater and surface water really is in comparison to rainwater.

Availability of groundwater and surface water in relation to rainwater

This is a no brainer right? Rainwater is seasonal, we only get it for a few months in a year, in Nigeria for example we get it for about 6-7 months in a year whereas groundwater and surface water are readily available to us, just switch on your pumping machine and turn on your tap and you’ve got water available to use anytime, if you are thinking along this line, you are actually right, I’ll give you that but don’t blame me for calling you shortsighted too, let’s take a look at the table below.

Average renewal time for groundwater, surface waters and rainwater

Now let’s try a little demonstration, take two buckets of equal volume, let’s say 25L each. Fill one of those buckets to the brim with water, now take a smaller bowl of 0.5L and then fill it up with water taken from the first bucket that was filled with water and then empty the bowl into the second 25L bucket which was initially empty, repeat this steps every 1 minute for 5 minutes then in the 6th minute fill the 0.5L bowl with water from the second bucket which should already be filled with 2.5L of water by then and then empty it into the first bucket, if we keep repeating this process of taking water from the first bucket 5 times and emptying it into the second bucket before taking water once from the second bucket to empty into the first bucket, we’ll find out that the water in the first bucket will be empty at some point and it would take more time to refill it than it took to empty it since we are only refilling it once in 6 minutes while we were taking water from it once every 1 minute.

This simple demonstration is the situation we currently find ourselves in in the world, the average renewal time for both surface and groundwater is so long and we are currently taking water from those sources in larger quantities than it’s been replenished and this already has led to what is known as water stress which is a situation where the available water at a point is lower than the demand in some parts of the world. Obviously the average renewal time for rainwater as seen in the table above is very short compared to the other water sources, so are those sources more available for our use than rainwater is? Well they are for now, but for how long?

How should it be done?

As I mentioned earlier, rain water is something we’ve probably all done at some point in our lives and for most people the obvious problem would be the quality of water gotten which might be a little turbid and possibly filled with sediments and certain impurities which is in contrast to the good water quality of rainwater I described earlier, so the question isn’t how can it be done but rather how should it be done.

To get good quality rain water to serve your various water need, a good rain water harvesting system needs to be put in place, a good rain water harvesting system generally consists of five components and they are the catchment surface, gutters and downspouts, storage tanks or cisterns, treatment facility and delivery systems. Let’s see what role each of these components play in delivering good quality rainwater to us.

• Catchment surface: the catchment surface is basically the medium used for the interception of the rain before it hits the ground surface, your catchment surface could be the roof of your house or saucers which you’ve set up for the purpose of harvesting rainwater, in selecting a catchment surface for rain water harvesting systems you need to consider the type of material which the surface is made of so you don’t introduce contaminants into your rain water, generally, metal is a good material for the catchment surface, you also need to consider the slope of the catchment surface, the slope should be such that it allows water to runoff easily instead of trapping the rainwater meant to be collected, the size of the catchment surface should also be considered as this would determine the amount of rainwater you can successfully collect.

• Gutters and downspouts: these are the channels used for conveying the water from the catchment surface to the storage system, they are basically just a network of pipes used to collect the water from the catchment surface, the size of the pipes should be considered when designing this, you don’t want your pipes to be too small in diameter to convey your water from the surface, gutters and downspouts should also be equipped with screens, first flush diverters and roof washers which are components used to remove debris and dust from the rainwater before it gets to the cisterns.

• Storage tanks or cisterns: the storage tank is where the rain water that has been collected is stored, plastic tanks are mostly used for this purpose but concrete tanks, metal tanks and fiberglass tanks can also be used, in choosing a storage tank, the volume of the tank needs to be considered, you would want to choose a volume which can serve your water needs sufficiently, the volume would also depend on the amount of rainfall you get in that particular area as well as the size of your catchment area, you don’t want to choose a storage tank that is too large when you only get little amount of rainfall in your area and you also don’t want to choose a small storage tank when you get a high amount of rainfall in your area as that would mean that your rain water harvesting system would not be efficient, so before selecting the volume of your storage tank, you should take a look at the rainfall data available in your area to determine how much water you can successfully store, you should also consider where you would have your storage tanks, you would want to site it somewhere where it won’t be easy for contaminants to get to it.

• Treatment facility: this particular component can be optional depending on what you intend to use the rain water for, if you’ve chosen to include drinking and other domestic use like cooking and bathing as part of the reason you are harvesting rainwater, then you would need a treatment facility but if the water is simply going to be used for other secondary needs such as irrigation, groundwater recharge or flood prevention then you might choose not to include a treatment facility.
  If you are including a treatment facility then you would need to consider what kind of treatment processes you would be doing and this depends on the type of contaminant that would be present in your water, if you’ve included screens and first flushers in your gutters and downspouts, you need not to worry about debris in your rain water but the rainwater would need to go through filtration process to remove smaller particles which pass through the screens, you would also need to disinfect the water to destroy microbes that would be present in the water, you also need to take a note of any industrial activity in your area as the industrial gases they release into the atmosphere could have been dissolved by the rain on its way down, so you might also need to treat that in your rain water.

• Delivery systems: this could be done either through gravity or by a pump, it includes all piping system that supplies the water from the storage tank to the point of use.

Final thoughts

Earth is a planet with an abundant amount of water resources, barring an unforeseen catastrophic event, the earth would most likely never run out of water, but that doesn’t mean humans would always have fresh water available for our use, with the rate at which our groundwater and surface water sources are being depleted without leaving enough time for its renewal, water stress is a genuine problem which we are likely to face in the future with countries such as Yemen, Libya, Jordan, Western Sahara and Djibouti as one of the top 5 already facing this problem^[3], while more countries are likely to find themselves in this situation in the nearest future, therefore it is imperative to relieve the stress on our groundwater and surface waters, to do this, there is a need to reduce the amount of water being withdrawn from these sources, but with the growing population of humans and their ever increasing water needs, looking for an alternative source of water seems to be the best chance of relieving water stress and that is where rain water harvesting comes in.

Harvesting rain water for our use helps in reducing the amount of water withdrawn from those water sources thereby giving the sources more time for their renewal, while its other advantages which includes good quality water,the fact that it's inexpensive to set up and the fact that when properly utilized in an area, it could help in preventing floods resulting from rainfall makes it a very attractive option as one of the best alternative water source.

• REFERENCES
• Rainwater Harvesting
• Distribution of water on earth's surface
• Five countries with Greatest water scarcity issues
• Syed R. Qasim, Edward M. Motley, Guang Zhu, 2000 - "Water Works Engineering: planning, design and operation"
• Robin Clarke, 1993 - "Water: The international crisis". Cambridge (MA): MIT Pres
• Rainwater Harvesting Components